1. Field of the Invention
The present invention relates to a satellite communication system and method for enabling ultra small aperture receivers by transmitting multiple spectral replicas to ultra small aperture receivers that coherently combine in frequency and phase of the desired signals of the multiple spectral replicas. More particularly, the present invention enables ultra small aperture communications systems by boosting power density (by replication) to the receiving antenna enabling ultra small aperture antennas in frequency bands like C band and Ku band.
2. Background of the Related Art
Satellite communications systems typically comprise an earth station (called a “hub”) and multiple geographically disperse smaller receiving antennas. Signals from the hub are transmitted on an uplink signal to the satellite, and retransmitted from the satellite to various smaller remote stations. The remote stations typically have a Very Small Aperture Terminal (VSAT) antenna. The antennas acquire the downlink signal from a particular satellite as that satellite passes through the field of view for that antenna.
As the aperture of the remote station antennas reduces, the G/T (antenna Gain/system noise Temperature) of the antenna degrades. This, in turn, reduces the effective difference between the desired signal and thermal noise and interference (C/(N+I)). As the antenna get smaller, the satellite's available power expended in the desired signal does not overcome the thermal noise and interference. Thus, usage of an ultra small aperture antenna is impractical.
In addition, as the aperture size decreases, the size of the beam gets wider. As the beam size increases, the field of view increases and the antenna is more likely to encounter more signals from multiple satellites that use the same frequency band (satellites are spaced as close as 2.5 degrees). As a result, VSAT antennas (with aperture sizes in the range of about 1.8-4.5 meters for C band and 9 cm-2.4 meters for Ku band) are susceptible to encountering Adjacent Satellite Interference (ASI). Such undesired signals interfere with the reception of the desired signal.
In practice, it is not feasible to utilize low-cost ultra small aperture terminal antennas (as small as about 80 cm for C band and 20 cm for Ku band). This is due to negative effects of low aperture resulting in an increase in ASI interference and a low G/T.
In addition, satellites have limited power and consequently have limited amounts of power which can be used to communicate uplink and downlink signals. Increasing power available on a satellite (i.e., its Equivalent Isotropically Radiated Power (EIRP)) can be very expensive. Moreover, if all (neighboring) satellites increased EIRP, the relative level of ASI (associated with using a small aperture antenna) would not reduce. However, the performance of a satellite communication link is proportional to the satellite power allocated to it. As link power increases (at additional cost), so does the link performance.
Thus, it is important to identify a controlled means of improving satellite communications systems, in particular (but not limited) to Fixed Service Satellites (FSSs) operating in lower frequency bands with 2 to 3 degrees of spacing between the satellites to allow the use of low-cost ultra small aperture terminals (for example, in improving the quality and reducing cost of DTH, i.e., Direct To Home, services).